The present invention relates in general to link grouping in packet switches for packets with constant length, e.g., ATM switches (ATM=Asynchronous Transfer Mode). Link grouping means that a logical link is created consisting of many parallel physical links. The logical link offers a bandwidth that is the sum of that of the physical links. In this way, for example, a logical link with a bandwidth of 622.08 Mb/s can be provided through link grouping of four physical 155.52 Mb/s links.
More particularly the invention relates to a method for providing link grouping in a packet switch of the kind intended to be used for constant length packets and between switch ports being divided into a number of nodes and transmission links, where the nodes perform space selection and the transmission links offer point-to-point transmission between the nodes,
said link grouping comprising creating, out of a number of parallel physical links coming into the switch, link groups, each in the form of a logical link with a bandwidth that is the sum of the bandwidths of the incoming physical links included in the link group, and restoring said logical link to outgoing parallel physical links from the switch.
In an ATM-switch for wideband telecommunication there is a need for higher bandwidth than that which is physically available. Link grouping can offer good traffic characteristics and hardware flexibility and effectivity, for example smaller delay, fewer buffers, and the realization of different types of concentrators/multiplexers and switches in different speed classes while using the same base components. Blocking characteristics in a link coupled network, for example a Clos-network, are also improved through logical speedup, achieved with the help of link grouping. Methods available thus far for offering a wideband link are:
1. high physical speed,
2. series-parallel conversion in order to attain higher logical speed than the physical one.
Disadvantages with the first method are the difficulty in realizing high speed electronics and mechanics, as well as the consequent power dissipation. The other method builds normally on multiplexing at bit-level and therefore requires bit-synchronous transmission.
Among publications discussing the need to create greater bandwidth than that which is physically available can be mentioned EP 0 374 574, WO 85/04300, WO 90/12467, and IBM Technical Disclosure Bulletin Vol 32, No. 9A, February 1990, pp. 45-49.